In many harsh environment oil-filled and pressure balanced cable-to-connector terminations the ends of cables penetrate an oil-filled compensation chamber. Boot seals are used within the compensation chamber to seal electrical conductor junctions and the area where individual conductors break out from the cable end. In addition to providing an electrically insulating barrier, the seals are intended to keep the oil from escaping into the cable's interstices. Such oil loss can quickly lead to chamber collapse and catastrophic termination failure. Prior art elastomeric breakout boot seals are easily displaced from their sealing position, and therefore do not have the reliability required for many applications. There is, therefore, a need for boot seals that remain in sealing position on the cable end to which they are installed.
Examples of both simple and breakout elastomeric boot seals can be seen in the subsea product offerings of DP Seals, Ltd. A typical prior-art cable-end, elastomeric, breakout boot seal is illustrated in FIG. 1. It consists of a sleeve 290a which is constrictively stretched over the end of cable 3. The environmental fluid pressure Pf, which is the fluid pressure external to the boot seal, has the effect of unseating the rear portion of sleeve 290a from cable 3. However, there is a pressure Pf Ps, where Ps is the “stretch” pressure of constrictive elastomeric sleeve 290a upon cable 3. The stretch pressure Ps works in cooperation with the external pressure Pf to keep the sleeve 290a seated. Since (Pf+Ps)≧Pf in all cases, the rearward portion of sleeve 290a will not be unseated by external pressure Pf, and the seal will not fail in that mode no matter how great the external pressure Pf. The same reasoning is true for all elastomeric boot seals wherein there is adequate stretch to conformably seat the sealing sleeve to the object over which it is stretched.
Individual conductor sleeves 250, which are integrally molded onto the heavy end-wall 490 of sleeve 290a, stretch over individual cable jacketed conductors 260. The same mechanism that worked to keep the interface between sleeve 290a and cable 3 sealed, keeps the interfaces between conductor sleeves 250 and jacketed conductors 260 sealed. That is, (Pf+Ps1)≧Pf, where Ps1 is the stretch pressure that conductor sleeves 250 exert on respective jacketed conductors 260. It is assumed for purposes of this discussion that Ps1 is the same for each of conductor sleeves 250, although in an actual application each could be designed to be different. In any case, it is clear that no matter how great external pressure Pf is, the various interfaces will remain sealed from the outside environment as long as the boot seal remains in position on the cable end.
Elastomeric breakout boot seals like that shown in FIG. 1 can be easily displaced and unseated in the case of rough handling or, as discussed in U.S. Pat. No. 6,796,821 (“the '821 patent”) incorporated by reference herein, if there is an overpressure within the cable to which they are attached. Such cases of unseating due to overpressure within the cable are more likely when external pressure Pf is small. In the case of subsea cables, that means either when the cable is not submerged, or when it is in shallow water. Some overpressure within the cable can occur even in light-duty operations due, for instance, to gas expansion within the cable itself. There are process-type breakout boot seals, such as the adhesive heat-shrink products manufactured by Tyco-Raychem, which sealably adhere to the cable, and are not as easily displaced. But using process-type boot seals is not always practical; for instance, when the cable jacket cannot be adhered to, or when the seal must be installed in conditions detrimental to the sealing process.